Recently, from the viewpoint of increase in environmental consciousness, liquid fuels in which the contents of environmental load substances such as sulfur and aromatic hydrocarbons are small have been demanded. From such a viewpoint, as a technique which can produce a base stock for fuel oil that substantially contains neither sulfur nor aromatic hydrocarbons and is rich in aliphatic hydrocarbons, particularly, a base stock for kerosene and gas oil, a method using a Fischer-Tropsch synthesis reaction (hereinafter, also referred to as the “FT synthesis reaction” in some cases) in which carbon monoxide gas and hydrogen gas are used as raw materials has been examined (see Patent Literature 1, for example).
A synthetic oil obtained by the FT synthesis reaction (hereinafter, also referred to as the “FT synthetic oil” in some cases) is a mixture containing aliphatic hydrocarbons with a wide carbon number distribution as a main component. From this FT synthetic oil, a naphtha fraction containing mainly a component with a boiling point of lower than 150° C.; a middle distillate containing mainly a component with a boiling point of about 150° C. to about 360° C.; and a wax fraction containing mainly a hydrocarbon component heavier than the middle distillate (with a boiling point of higher than about 360° C.) (hereinafter, referred to also as the “FT wax fraction” in some cases) can be obtained. Among the respective fractions, the middle distillate is the most useful fraction corresponding to a base stock for kerosene and gas oil and is desired to be obtained with a high yield. Therefore, in an upgrading section of hydroprocessing and fractionating the FT synthetic oil to obtain a base stock for fuel oil, the FT wax fraction produced in a significant amount with the middle distillate in the FT synthesis reaction step is converted to a component corresponding to the middle distillate through the hydrocracking to reduce molecular weight, thereby enhancing the yield of the middle distillate as a whole.
The FT wax fraction obtained from the FT synthetic oil by fractionating is hydrocracked in a wax fraction hydrocracking reactor packed with a hydrocracking catalyst, and then separated into gas and liquid in a gas liquid separation apparatus. Then, the liquid component thus obtained (hydrocarbon oil) is sent to a fractionator at the following stage along with the middle distillate preliminarily fractionated from the FT synthetic oil and separately hydrotreated, and the middle distillate (kerosene and gas oil fraction) is obtained by fractionating. In this case, a heavy component containing an uncracked wax (bottom oil) which has not been sufficiently hydrocracked in the wax fraction hydrocracking reactor is recovered from the bottom of the fractionator. The entire amount of the bottom oil is recycled, fed to the wax fraction hydrocracking reactor again along with the FT wax fraction from the FT synthesis reaction section, and hydrocracked (see Patent Literature 2, for example).
While an operation is performed in the wax fraction hydrocracking reactor with a cracking rate described later being set so as to maximize the yield of the middle distillate, the hydrocracking catalyst packed in the reactor degrades and an activity thereof decreases with the time on operation, and the yield of the middle distillate reduces with the decrease in cracking rate. Then, adjustments are usually made so that the cracking rate is constantly maintained by raising a reaction temperature over time for compensating for the decrease in activity of the catalyst. This reaction temperature rises with the passage of operation time, an apparatus is stopped at the time of reaching the upper limit temperature of the catalyst or apparatus, and replacement or regeneration of the catalyst is performed. Therefore, in order to improve a production efficiency of the hydrocarbon oil, there is a demand for making it possible to suppress the degradation of the hydrocracking catalyst over time and use the catalyst continuously for a longer time.
As a device for suppressing the degradation of the hydrocracking catalyst and extending a continuous use time of the catalyst, a method has been proposed in which a raw material is temporarily switched from a wax fraction to light paraffin hydrocarbons at the stage that the degradation of the catalyst over time is caused, an activity of the catalyst is allowed to be recovered, and thereafter the raw material is returned to the wax fraction (see Patent Literature 3, for example).